The present invention relates to moving platforms, specifically, to a system for determining the geodetic attitude of an arbitrarily moving platform. Moving platforms include vehicles, such as aircraft, ground vehicles, boats or spacecraft or equipment mounted to vehicles that can be reoriented relative to the vehicle body. The platforms may be traveling at fast or slow speeds, may be maneuvering or non-maneuvering, and may be occasionally stationary relative to geodetic space. These platforms require knowledge of their geodetic attitude in order, for example, (i) to support safety or stability control systems, (ii) to point an antenna or sensor boresight at a geodetically known target, (iii) to control their geodetic position or attitude movement, or (iv) to register the information sensed along the boresight onto a map projection with geodetic coordinates. The sensor or antenna boresight is the centerline of some signal collection or signal transmission aperture.
Earth-rate sensing through gyrocompassing, GPS interferometry, and transfer alignment (TA) are possible implementation approaches for precision geodetic orientation measurement systems for arbitrary moving platforms. Each technique is in widespread use, but each technique alone has significant limitations for precision pointing.                Earth rate sensing requires the use of a gyroscope with accuracy much better than the earth's 15-deg/hr-rotation rate. The gyroscopes used for conventional gyrocompass systems have drift specifications of less than 0.1 deg/hr. Modem military gyroscopes, currently used on missiles, can achieve a 1 deg/hr accuracy with prices of about $5000 in large quantities. For a 1-deg/hr tactical weapon grade gyroscope, the north seeking accuracy is about 4 deg and is not sufficiently accurate for broadband pointing.        GPS interferometry measures GPS carrier phase to GPS satellites from multiple spaced antennas. Carrier phase differencing removes all common mode ionospheric corruption from the differenced signals. The remaining phase difference can be used to infer range to GPS satellites to millimeter (mm) accuracy. The measurement is corrupted by cable-induced phase differences, on-vehicle multipath, and a whole-cycle GPS wavelength ambiguity that is 19 cm for commercial GPS. A method not based on interferometry is often used to get close to the correct attitude and reduce whole-cycle ambiguity. Commercial motion characterization systems that use GPS interferometry are available, but impose installation difficulties by requiring multiple antennas dispersed over several square meters. Also, the lack of wide-bandwidth attitude memory prevents any accuracy enhancement through data averaging unless the system is perfectly stationary.        Transfer alignment is the most widely used precision orientation measurement method for military applications. Transfer alignment synergistically combines an Inertial Navigation System (INS) with single-antenna GPS system to estimate position and attitude. The INS, traditionally used only in military applications and high-end aircraft, has an internal instrument suite that provides measurement of three axes of acceleration and three axes of rotation rate. Mathematical manipulation of the acceleration and rotation rate measurements provides the position, velocity, and attitude of the platform at a high bandwidth. However, the navigation solution will drift unless some external corrections are incorporated. For low cost inertial components, the drift will occur rapidly. GPS external measurement is most often used for the INS corrections. For GPS transfer alignment, INS-derived velocity and GPS-derived velocity are differenced, and the time-history of the differences is then used to infer errors in assumed geodetic alignment of the INS axes. The need to maintain persistent changing velocity to enable attitude measurement and the traditional high cost of the INS make TA unsuitable for most commercial applications. TA uses a mathematics model where attitude errors propagate into the IMU-derived platform position and velocity in geodetic coordinates. By independently measuring the geodetic position and velocity with the navigation GPS solution, the attitude errors are observed and corrected. However, the attitude errors are observable through the velocity, such that a change in attitude produces a change in geodetic velocity. The presence of a specific force acting on the platform must be present to impart attitude observability. A specific force is almost always present in the vertical direction since a force must be imposed to maintain the platform from falling towards the center of the earth. Thus, attitude orthogonal to the vertical direction, the platform roll and pitch angles, are readily observed for any platform not in a free-fall condition. However, a platform at a constant velocity in the horizontal plane will have no observability of attitude about the vertical direction, the platform yaw angle. For successful transfer alignment, the horizontal plane motion must be sensed by the GPS carrier phase measurements from the navigation GPS antenna. Because integrated carrier phase measurements are accurate to millimeter (mm) levels, even for a very low cost commercial receiver, only a slight platform motion is sufficient for some level of heading attitude measurement. Most moving platforms will have some motion from external disturbances for attitude estimation to an accuracy of several degrees.        
In addition to the techniques just described, the prior art also includes patents teaching techniques to determine geodetic attitude from moving vehicles. For example, U.S. Pat. No. 5,575,316 to Buchler describes a generalized motion characterization system employing multiple GPS antennas and receivers and an IMU device. Key to Buchler's preferred embodiment of this device, and clearly stated throughout his sample embodiment and claims, is a requirement to overcome a large uncertainty in the initial platform heading. This initial heading error, coupled with a 1 m GPS antenna spacing, causes the GPS interferometric range-to-satellite ambiguities to produce ambiguous platform heading measurements. The large initial heading error of 10 deg stated by Buchler results from the use of gyrocompassing to ascertain initial platform heading independent of GPS interferometry. Gyrocompassing, as is understood in the art, relates to sensing the rotation rate of the 15-deg/hour-earth vector. The LN-200 IMU rate gyro employed in the Buchler invention has a gyro drift of 1 deg/hr that produces the 10 deg heading error following a gyrocompass event. The majority of the Buchler invention relates to the refinement of the initial attitude uncertainty to a level where no ambiguities are present in the final measurement.
The Buchler invention poses at least six considerations that prevent low manufacturing cost, ease of installation, and operation with arbitrary platforms:                Two independent GPS receivers are required to determine double difference relationships used for the interferometric processing. This means that two oscillators are used in the GPS RF front-end downconversion process. The use of two oscillators causes added measurement noise when carrier phase from the two channels are differenced. Also, the use of two independent GPS receivers increases the cost.        The use of a 1 deg/hr IMU, necessary for gyrocompassing attitude initialization, demands a relatively high-cost IMU unsuitable for most commercial applications.        The use of a gyrocompass stage to initiate the attitude measurement process is not suitable for arbitrary platform operations. Gyrocompassing restricts the platform motion and requires a significant period of initialization time.        The Buchler invention assumes the use of a barometric altimeter for independent altitude measurement. Such a measurement is problematic for all platforms because of the need to maintain a clean and precisely oriented passage to ambient airflow.        The Buchler invention assumes that all GPS satellites visible on one antenna are also visible to the second antenna to arrive at the double differences used by a Kalman filter. This suggests a requirement for use of either two standard GPS receivers, each tracking the same GPS satellites, or specialized, more costly, receiver architecture with twice the standard number of channels.        A problem is posed by the Buchler invention that relates to achievable accuracy of the attitude solution. The bulk of the embodiment relates to the use of a double-difference phase function, which Buchler claims to treat as a scalar measurement to the Kalman filter. Double differences result in M-1 scalar measurements for M GPS satellites being tracked. However, the measurements are correlated because common GPS satellite ranges are used in multiple measurements. Treating such correlated measurements as uncorrelated scalar measurements by the Kalman filter leads to a suboptimal filter, as is well known in the art. The embodiment mentions the use of a single-difference measurement formulation but does not describe how this mechanization will produce uncorrelated scalar measurements for the Kalman filter.        
In another patent example, U.S. Pat. No. 5,617,317 to Ignagai describes a generalized motion characterization system employing multiple GPS antennas and receivers and an IMU device. The Ignagai invention assumes the existence of a separate Inertial Sensor System on the platform, distinct from the dual-antenna GPS system. Ignagai does not fully integrate the IMU rotation rate and acceleration measurements into the attitude measurement processing; instead, the Ignagai invention takes independently derived attitude information from the Inertial Sensor System and combines it with differential range information determined from a two-antenna interferometric GPS system. Ignagai uses a simple three-state Kalman filter to smooth the angular misalignment between the two independently derived heading angles. As in the Buchler invention, two independent GPS antenna/receivers are used coupled to a differential range processor. The Inertial Sensor System is said to be an Attitude and Heading Reference System (AHRS), which is known with the art to be a self-contained navigation system employing a separate air-data system, as explained by Ignagai.
Ignagai describes three types of interferometric measurement processing: differential range, differential carrier phase, and differential integrated Doppler counts. Ignagai discusses antenna separations of 10–20 m for the differential range measurement, 1–2 m for the integrated Doppler count method, and “a possibility” of 3.75 inches separation for the differential carrier phase measurement. However, the embodiment develops only the formulations for the differential range and the integrated Doppler count methods. Ignagai makes little mention of the interferometric heading ambiguity problem treated extensively by Buchler.
Ignagai discusses the heading initialization as using the aircraft cockpit magnetic compass for a stationary aircraft, or by using the aircraft track heading while the aircraft is taxiing on the ground. The aircraft track heading initialization process assumes that the IMU and antenna baseline are aligned with the taxi velocity so that the heading alignment is equal to the ground velocity vector as measured from GPS. Ignagai notes that this is problematic for an in-air initialization of the heading because the aircraft body attitude is not aligned with the velocity vector.
Seven considerations are posed by Ignagai that prevent achieving low manufacturing cost, ease of installation, and ease of operation with arbitrary platforms:                Two independent GPS receivers are required to determine the interferometric relationships used for the interferometric processing. This means that two oscillators are used in the GPS RF front-end downconversion process that contributes to the phase measurement noise. Also, two GPS receivers increase the cost.        Ignagai assumes a separate and distinct Inertial Sensor System, such as an AHRS, that will be too costly for general commercial applications.        The use of an aircraft track heading procedure for initializing the heading measurement process is not generally suitable for platforms where the IMU and GPS baseline are arbitrarily oriented with respect to the platform velocity vector.        Ignagai assumes the use of an Air Data Sensor. Such a measurement sensor is problematic for all platforms because of the need to maintain a clean and precisely oriented passage to the ambient airflow.        Ignagai assumes that all GPS satellites visible to one antenna are also visible to the second antenna to arrive at the interferometric differences used by the Kalman filter. This suggests the requirement for either using two standard GPS receivers that each tracks the same GPS satellites or using a tailored receiver architecture with twice the standard number of channels.        Ignagai uses a simple three-state Kalman filter for smoothing the inertial sensor and GPS interferometric angle errors. Such a simplistic filter form cannot exactly represent the precision attitude memory achievable when a more complete IMU and GPS integration is mechanized. This prevents the optimal merging of past interferometric measurements and restricts the achievable measurement accuracy.        Ignagai integrates the Inertial Sensor System and interferometric range system through a filter applied to a Euler angle. This approach results in a mathematical problem as the system crosses the earth poles. A coordinate system switch is required as the platform reaches higher latitudes. This is undesirable and reduces the generality of the invention for general platform geodetic motion.        
Two more patent examples include U.S. Pat. No. 5,672,872 to Yeong-Wei and U.S. Pat. No. 5,809,457 to Yee. Both Yeong-Wei and Yee describe a generalized motion characterization system employing a GPS antenna and receiver integrated to an IMU device via a Kalman filter. Both Yeong-Wei and Yee inventions use a single GPS antenna rather than the dual antennas of the Buchler and Ignagai inventions. Yeong-Wei specifically describes the well-known problem of such single-GPS-antenna mechanizations: persistent maneuvers are required to enable the heading attitude to be observable. Purposeful aircraft maneuvers are described as necessary for the example aircraft embodiment. Yee is specialized to an application where the GPS antenna and IMU are located to the boresight of a sensor or antenna system. However, Yee makes no reference to the problem of heading errors when persistent horizontal plane maneuvers are not present. Yee makes no mention of intentional maneuvers for achieving the heading alignment. Neither Yeong-Wei nor Yee mentions the use of dual GPS antennas for the purpose of avoiding the heading drift when horizontal plane motion is not present.